We have studied by molecular hybridization the genome of the Friend strain of spleen focus-forming virus (SFFV), a replication-defective, highly leukemogenic virus which arose by recombination between ecotropic F-MuLV and endogenous murine sequences which are highly related to xenotropic virus. The xenotropic-related sequences contained in SFFV have been found to be highly related to sequences in murine mink cell focus-inducing viruses (MCF viruses), and were derived from the env gene region of a xenotropic virus. Finally, we have isolated an MCF recombinant virus derived from F-MuLV and have shown that SFFV codes for a gene product which is cross-reactive with gp70 derived from this Fr-MCF virus.